Slip casting of tungsten

ABSTRACT

AN IMPROVED METHOD OF MAKING A TUNGESTEN SLIP CASTING SLURRY IS DISCLOSED. THE SLURRY MAY CONTAIN ONLY WATER AS THE SUSPENDING AGENT DUE TO THE CONTROLLED DENSITY AND SIZE OF THE TUNGSTEN POWDER.

United States Patent 3,578,478 SLIP CASTING 0F TUNGSTEN Craig T. Waldo, Towanda, Pa., assiguor to Sylvania Electric Products Inc. No Drawing. Filed Sept. 16, 1968, Ser. No. 762,327 Int. Cl. C08h 17/24; B22f 3/22 US. Cl. 106286 6 Claims ABSTRACT OF THE DISCLOSURE An improved method of making a tungsten slip casting slurry is disclosed. The slurry may contain only water as the suspending agent due to the controlled density and size of the tungsten powder.

BACKGROUND OF THE INVENTION This invention relates to a tungsten powder that enables an improvement to the slip casting of tungsten articles. More particularly it relates to an improved process for making slip casting slurries to be used in the production of tungsten articles.

To overcome the limitations and difficulties encountered with conventional metal processing techniques in the manufacture of articles produced from tungsten, slip casting techniques have been developed. These processes, which generally involve the formation of an aqueous slurry of tungsten powder, and thereafter the sintering of the slurry to form cast tungsten articles, overcome the metal processing difficulties, however, excessive process times are required to for-m a powder containing tungsten that will produce an aqueous slurry that can be cast and subsequently sintered to form high quality tungsten articles. For example, a process is disclosed in US. Pat. 3,322,536 that produces a tungsten oxide or carbon coated powder and the powder is thereafter slurried with water. The time required in the abovementioned patent is at least about five hours. Additionally it is to be noted that even after the tungsten is oxidized in a ball mill for the long period of time, in order to obtain a suitable slurry, it is necessary to add a minor proportion of a wetting agent. It is believed that a tungsten powder that can be produced in a relatively short period of time and is suitable for slip casting slurries that are sintered to form various tungsten articles is an advancement in the art.

SUMMARY OF THE INVENTION In accordance with one aspect of this invention there is provided a tungsten powder having a maximum particle size of about five microns and an average particle size of less than about 0.8 microns and a bulk density of greater than about 35 grams/cubic inch. According to another aspect of this invention a mixture comprising water and the beforementioned tungsten powder having a water to powder ratio of about 1:4 to about 1:10 permits the formation of various slip cast articles of tungsten that are reasonably free of defects. An additional aspect of this invention enables the production of a slip cast tungsten article in a time period believed heretofore unachieved with materials previously known.

For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above description of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT S The present invention enables the use of water as a vehicle for suspending the tungsten rather than organic chemical vehicles or aqueous slurries to which a wetting ice agent or a deflocculant has to be added. It has been discovered that tungsten powder having discrete particle sizes and a minimum bulk density can be mixed with water to form an aqueous slurry that can be slip cast without separation of the particles. Furthermore, the present invention eliminates the need to resort to coating procedures which are, in general, time consuming.

Although the tungsten raw material used in the practice of this invention can have various particle sizes, it is generally preferred to start with a tungsten powder having at least a major proportion of the particles having a particle size of less than five microns. The powder is subjected to conditions for a time sufiicient to increase the bulk density of the powder to at least about 35 grams/ cubic inch. In most instances subjecting the above defined tungsten powder to the mixing obtained in mixers such as the Prodex Henschel type of mixer for about 30 seconds to about 3 minutes is sufficient to produce the tungsten powder of this invention. In most instances the bulk density of the starting material will be less than about 33.75 grams/cubic inch. Although tungsten powder having a maximum particle size of less than five microns and an average particle size of less than about 0.8 micron can be used, it is preferred to use smaller particle sizes, such as a maximum particle size of smaller than 4 microns and an average particle size of 0.7 micron or less. It is also preferred that less than about 30% of the particles are larger than about 1.0 micron and that greater than have a particle size less than 2 microns. It is also preferred that bulk density of the powder be about 35 grams/ cubic inch to about 40 grams/cubic inch.

Although the addition of other additives such as wetting agents, deflocculants, binders and the like do not have to be used, such additives can be employed if desired. In most instances when smaller articles that are below about five inches in depth are to be cast, the beforementioned additives do not appreciably add any beneficial properties to the slurry. If larger items, such as from about five to about twelve inches in depth are prepared, some additional benefits in the quality of the product can be obtained by adding some of the additives although the slurry is operable without the addition.

After the tungsten powder having the desired particle size and bulk density is produced, it is mixed with 'water to form a relatively uniform mixture. A water-to-tungsten ratio of from about 1:4 to about 1:10 is used to provide a slurry in which the tungsten does not settle, therefore, the use of other ratios of water-to-tungsten are not necessary. The preferred ratios of water-to-tungsten are from about 1:5 to about 1:6 in order to achieve the optimum quality of articles.

After the slurry is prepared having the beforementioned characteristics, the slurry is poured into the desired casting or mold. Generally the cast or mold containing the slurry is air dried for a relatively short period of time and thereafter sintered for about ten hours at a temperature of from about 1700 C. to about 1800 C. In some instances, particularly if the article is large, it can be desirable to presinter at a lower temperaturethat is from 1100 C. to about 1200 C. for a short period of time, before raising the temperature of the material to the beforementioned sintering temperature. For example, satisfactory results are achieved when the presintering is conducted at 1150 C. for a time period calculated on the basis of 8 minutes per inch of the largest dimension. The shrinkage of the article from its cast volume to the volume after sintering is about 23% to about 27% by volume. The density of the finished article will vary from about 17 g./cc. to about 18.5 g./ cc. depending upon the time the article is sintered and at the temperature at which the article is sintered. For example, a density of about 17 g./cc. is attained within about three hours and a density of about 18 g./cc. is attained within twelve hours when the articles are sintered at about 1800 C. If sintered at about 2200 C. for about 4 hours, a density of about 18.5 g./ cc. is achieved.

To further illustrate the preferred embodiments of this invention the following detailed examples are presented. All parts, proportions and percentages are by weight unless otherwise indicated.

EXAMPLE I Particle size (microns): Percent by weight -1 62.26 1-2 24.80 2-3 4.85

Average particle size 0.75 micron. 'Bulk density 36.4 grams/ cubic inch.

The powder is slurried with water using a 1:5 Water-totungsten powder ratio. A mold for casting a tungsten boat is filled with the slurry and the excess slurry is removed from the mold. The tungsten is allowed to remain in the mold for about one hour, then removed from the mold, air dried and sintered at about 1750 C. for ten hours. The tungsten boat is suitable for metallurgical analytical techniques where high temperatures are required for analysis.

If small holes are required in the tungsten article, such as 0.003 inch in diameter, small diameter combustible threads can be cast into the article and these will be removed during the sintering step leaving a hole of small uniform diameter. Additionally if desired, a wire can be inserted in the article at the time of casting and removed prior to sintering and the desired hole can be obtained.

EXAMPLE 11 Particle size (microns): Percent by weight 0-1 70.24 1-2 24.70 2-3 3.87 3-4 1.19 4-5 0 Average particle size .70 micron. Bulk density 38.1 grams/cubic inch.

Samples of the powder are slurried with water using a water using ratios of water to tungsten of varying ratios from about 1:4 to about 1:10. The slurries are used to fill molds and the excess slurry is removed from the mold. The material is allowed to remain in the mold for varying times from about 1 hour to about 4 hours depending upon the water to tungsten ratio. After the tungsten is removed from the mold the article is sintered at various temperatures for varying times. Articles are sintered at about 1750 C. for a time period based upon the size of the article. Calculated on the basis of 4 minutes per linear inch of the largest dimension of the article. The density of the sintered article is calculated to be about 17.9 g./ cc. Articles presintered at 1150 C. and the final sintering at about 2200 C. for a period of about 4 hours yield articles of a density of about 18.5 g./ cc. or 96% of theoretical.

While there have been shown and described What are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the appended claims.

I claim:

1. A process for preparing a finely divided tungsten powder suitable for Slip casting slurries comprising subjecting a tungsten powder having a major proportion of the particles smaller than five microns and a bulk density of less than about 34 grams/ cubic inch to high intensity mixing for a time sufficient to increase the bulk density to at least 35 grams/cubic inch and to produce a powder having an average particle size of at least 0.75 micron.

2. A process according to claim 1 wherein said time is at least about 30 seconds.

3. A process according to claim 2 wherein said time is from at least about 30 seconds to about 3 minutes.

4. A process for preparing a tungsten slip casting slurry comprising:

(a) subjecting a tungsten powder having a major proportion of the particles smaller than five microns and a bulk density of less than about 34 grams/cubic inch to high intensity mixing for a time sufiicient to increase the bulk density to at least 35 grams/cubic inch and to produce a powder having an average particle size of at least 0.75 micron; and

(b) forming an aqueous mixture comprising water and the tungsten powder with the increased bulk density, said mixture having a water to tungsten ratio of from about 1:4 to about 1: 10.

5. A process according to claim 4 wherein said time period is at least about 30 seconds.

6. A process according to claim 5 wherein said time period is from at least about 30 seconds to about 3 minutes.

References Cited UNITED STATES PATENTS 1,675,119 6/1928 Marden 264-86 2,763,541 9/1956 Mettler, Jr. -0.5 2,979,401 4/ 1961 Szymaszek 264-86 3,052,532 9/ 1962 Stoddard et a1 75-0.5 3,216,841 11/1965 Thellman 264--86 3,234,007 2/ 1966 -Blocher, Jr. et a1. 750.5 3,236,634 2/1966 Lambdin et a1 750.5 3,341,320 9/1967 Smiley 75-0.5 3,444,032 5/ 1969 Kreier, Jr. 2641 11 3,480,426 11/ 1969 Nevenschwander 75-0.5 3,475,158 10/ 1969 Nevenschwander 75-0.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner U.S. Cl. X.R. 75-0.5 

